A.I.S.I. Type 416 alloy is a hardenable martensitic stainless steel alloy that provides a higher level of machinability relative to other known grades of martensitic stainless steels. The ASTM, UNS, and AMS standard compositions for Type 416 alloy are as follows, in weight percent.
______________________________________ ASTM UNS AMS ______________________________________ C 0.15 max. 0.15 max. 0.15 max. Mn 1.25 max. 1.25 max. 2.50 max. Si 1.00 max. 1.00 max. 1.00 max. P 0.06 max. 0.060 max. 0.060 max. S 0.15 min. 0.15 min. 0.15-0.40 Cr 12.00-14.00 12.00-14.00 11.50-13.50 Ni -- -- 0.75 max. Mo 0.60 max. 0.60 max. 0.60 max..sup.1 Cu -- -- 0.50 max. Fe Bal. Bal. Bal. ______________________________________ .sup.1 Mo or Zr
Modifications to the basic Type 416 alloy have been made to improve it machinability by including a positive addition of manganese or a combination of tellurium, aluminum, and copper. While those elements are known to benefit the machinability of Type 416 stainless steel, they are also known to detract from such desirable properties such as corrosion resistance and processability when present in too great amounts. Processability relates to the hot workability and ease of melting of the alloy. In addition, the inclusion of such elements in the basic alloy composition results in an alloy that is outside the industry-accepted compositional limits for Type 416 alloy. Customers and potential-customers for Type 416 alloy are reluctant to purchase such modified grades because of uncertainty about the effects of the compositional modifications on the desired properties for the Type 416 alloy other than machinability.
U.S. Pat. No. 3,401,035 relates to a free-machining stainless steel that is based on the Type 416 alloy. That patent discloses that increasing the chromium-equivalent, and hence the amount of ferrite, in the alloy is beneficial to the drilling machinability of the Type 416 alloy. However, the presence of too much ferrite in a martensitic stainless steel such as Type 416 adversely affects the hardness capability of the alloy such that the high levels of hardness and strength typically specified for that steel are not attainable. The patent indicates that "the principles of the invention" described therein "are equally applicable to steels having either a duplex or ferrite-free microstructure." However, there is no discussion of how the composition of such alloys should be balanced to provide a significant improvement in form tool machinability.
In view of the foregoing, there is a need for a martensitic stainless steel alloy that provides improved form tool machinability relative to the known grades, but which provides at least the same level of hardness capability and corrosion resistance as those grades.